This invention relates generally to a liquid crystal display device utilizing thin film transistors (TFT) and more particularly to a liquid crystal display having storage capacitors of adjacent row or column liquid crystal cell circuits connected to a common electrode comprising commonly connected capacitor lines.
An example of a liquid crystal display device utilizing TFT switching devices is described in an article to Matsueda et al., entitled "Optimization of 0.94-in. Poly-Si TFT Light Valve for LCD Projector", The Japan Display 89 Digest, pages 418-421, Date, 1989. As an example of the display element circuit of Matsueda et al. employing TFT switching devices is illustrated in FIG. 2. The column signal lines are X.sub.1, X.sub.2. . . to which image signals are applied and the row scan lines are Y.sub.1, Y.sub.2, Y.sub.3, Y.sub.4. . . to which TFT switching signals are applied. TFT devices 11, 12, 13 are provided at the cross point between signal line X.sub.1 and scan lines Y.sub.1, Y.sub.2, Y.sub.3. . . adjacent to these respective devices. TFT devices 11', 12', 13' are provided at the cross point between signal line X.sub.2 and scan lines Y.sub.1, Y.sub.2, Y.sub.3. . . adjacent to these respective devices and so on. TFTs 11, 12 and 13 are turned ON or OFF in accordance with the timing of a selective pulses applied to respective scan lines Y.sub.1, Y.sub.2, Y.sub.3. . . . The image signal applied to signal line X.sub.1 is written into respective liquid crystal display (LCD) elements represented by capacitances 17, 18 and 19 and storage capacitors 14, 15 and 16 through a respective activated TFT 11, 12 and 13. Thus, for example, TFT 11, storage capacitor 14 and LCD element 17 represent one pixel structure.
FIG. 3 is a plan view showing an example of a pixel portion of the liquid crystal display device of FIG. 2. TFT 24 is arranged in the crossing point between signal line 21 and scan line 22. Semiconductor thin film 23 forms the source/drain and channel for TFT 24. Contact hole 25 provides for connection of the source of TFT 24 with signal line 21, and contact hole 26 provides for connection the drain of TFT 24 with pixel electrode 29. Storage capacitor 28 is formed between the drain of TFT 24 and common electrode 27. In the case of a transmissive type display device, the portion indicated as aperture 30 is the image display region for a pixel structure.
The pixel structures are required to be fabricated with high density in order to achieve high resolution quality images. However, a decrease in pixel pitch results in a corresponding decrease in capacitance of the liquid crystal and, therefore, it is necessary to form adjacent storage capacitors to provide for sufficient capacitance to produce the same high image resolution quality.
Furthermore, miniaturization relative to the size of the TFT devices is limited by the manufacturing process. Therefore, when the pixel pitch of the LCD elements is decreased, ratio of the regions occupied by the combination of the storage capacitor and TFT become correspondingly larger, and the ratio of the regions occupied by the image display apertures must become correspondingly smaller. A decrease in aperture ratio correspondingly results in increased darkness in the display and, further, the image quality and resolution is significantly reduced since the light shielding pattern surrounding the display apertures becomes larger so that the displayed image has a thick lattice appearance.
It is an object of this invention to provide a projection type liquid crystal display device to solve the above mentioned problems relative to image darkness and quality.
It is another object of this invention to provide a liquid crystal display device having a high aperture ratio with decreased pixel pitch.